The publication in 1992, by the International Standards Organization, of ISO standard 9223 on the corrosivity of atmospheres represented an extraordinary advance in classification of the aggressivity of atmospheres from the viewpoint of metallic corrosion in outdoor exposure. In 1988, basically following the experimental methodology of the ISOCORRAG programme of ISO/TC156/WG4, the MICAT project began. This was a four-year programme of field research on atmospheric corrosion, conducted both in laboratories and in a network of 75 atmospheric exposure test sites throughout the Ibero-American region, thus considering a broad spectrum of climatological and pollution conditions. This work compares, for the four reference materials (carbon steel, zinc, copper and aluminium), the corrosion data obtained in the ISOCORRAG and MICAT programmes, as a function of the environmental variables of the different atmospheres (time of wetness and deposition rates of SO2 and chlorides), analysing the corrosivity categories obtained and those estimated on the basis of ISO standard 9223. In a good number of cases deviations were shown in comparison with ISO standard 9223. The paper makes recommendations for a possible future revision of ISO 9223, with the aim of making a better estimation of atmospheric corrosivity categories on the basis of environmental variables.